My youngest daughter has Angelman Syndrome. With this disorder comes severe mental retardation. She is able to understand more of what I say to her, but has trouble getting things back out. Her school (for special needs children) uses sign language daily. After doing a little research I found out that babies can even pick up on signs before they can even learn to talk.
I needed to learn how to sign, but couldn't attend a class. That's when a friend gave me her The Joy of Signing book. I love this book at it is complete with over 1500 signs that are illustrated beautifully. What I really like about this book is that all the signs are grouped in like categories. For instance you can turn to the family relationships page to learn how to sign Mom, Dad, sister, Grandmother, etc. It would be a pain in the neck if you had to look up each sign alphabetically as you would generally want to learn how to sign Mom and Dad at the same time.
This book's illustrations are so easy to understand that my eight year old daughter can pick up on them right away. I do believe a person could learn how to sign fluently with this book without ever having taken a sign language class.
This book gets an A+ from me and my girls.